1. Field of Invention
This invention relates generally to electronic systems and more particularly to cooling systems for use in electronic systems.
2. Discussion of Related Art
Electronic systems contain components that consume power, much of which is dissipated as heat. For many electronic systems, the packaging density of the electronic components is low enough that the heat generated as the system operates can be dissipated into the air surrounding the electronic system. For systems that dissipate large amounts of power as heat or that are packaged into smaller areas, cooling fans are often used. The fans enhance the airflow across hot components to increase the rate at which heat is dissipated into the air.
As the heat density of electronics systems increases, either because the components in the system generate more heat or because the components are packaged into smaller areas, air cooling is not able to keep the electronic system operating in a desired temperature range. To provide greater cooling, some systems have used water or other fluids with a higher specific heat than air. In such systems, a cold plate is often positioned next to electronic components that generate substantial heat. The cooling fluid circulates through passages in the cold plate. The temperature of the fluid increases as it absorbs heat from the electronic components. The fluid is circulated to a heat exchanger away from the electronic circuitry. The heat exchanger is used to dissipate the heat from the fluid into the air. The cooled fluid is then recirculated through the cold plate, where it again absorbs heat from the electronic components. An example of such a system is given in co-pending U.S. patent application Ser. No. 10/741,542, filed on Dec. 19, 2003 and entitled MODULAR RACKMOUNT CHILLER (now U.S. patent application publication 2005/0133214), which is hereby incorporated by reference.
The fluid temperature of the before-mentioned cooling system will depend on the heat load. As a result, the fluid expands and contracts. If the system is left freely vented to the outside atmosphere, fluid loss will result. Therefore these types of cooling systems are typically sealed from the outside to minimize fluid loss due to these thermal expansion effects, and also due to direct evaporation if it were left freely venting to the outside ambient environment. However, to avoid excessive pressures in the system, a pressure-relief valve is used to limit the pressure in the system typically to about 10 psig at the tank location. When the system vent opens, vapor and entrapped air escape. As a result of the pressure-relief valve, the undesirable loss of fluid is reduced but not eliminated. In addition, a vacuum-relief valve is used to limit the minimum pressure to about −5 psig.
Such cooling systems in which heat is removed from a system by a temperature increase of a fluid are often called single-phase cooling systems, where the heat removed based on a temperature change is called sensible heat. The amount of heat removed by a single-phase system depends on the mass flow rate of the cooling fluid, its specific heat capacity, and its allowable temperature rise. There are practical limits on the sensible heat removal in a cooling system and also on the rate of flow of such fluids.
Where greater cooling than can be provided with a single-phase system is desired, two phase cooling systems are sometimes used. In a two-phase system, the cooling fluid is allowed to boil or evaporate. The fluid thus undergoes a phase change. When a fluid changes its state from a liquid to a vapor, it absorbs heat in proportion to a material property known as the “latent heat of vaporization” of the material. The latent heat is generally much larger than the sensible heat. Thus, by allowing some or all of the cooling fluid to undergo a phase change, the amount of heat removed from the electronic system can be greatly increased or the mass flow rate of the fluid can be greatly reduced, allowing for a smaller cooling system.
Most two-phase cooling systems are closed loop systems, meaning that the vapor from the cooling fluid is condensed back to a liquid and recirculated for further cooling of the electronic system. Most closed loop cooling systems therefore include a condenser or other form of heat exchanger. An example of a closed loop two phase cooling system is described in U.S. Pat. No. 6,519,955 to Marsala, which is hereby incorporated by reference.
We have recognized a drawback in using available two phase closed loop cooling systems with electronic systems. Electronic systems are often assembled from printed circuit boards or other subassemblies that are plugged into a rack or similar structure. A backplane or other interconnection structure makes the electrical connection between the printed circuit boards. For liquid cooling, a cold plate is often attached to each printed circuit board. A manifold or other fluid distribution network built into the card cage runs fluid to and from the cold plates. Quick disconnect couplings are often used to connect the cold plates on the printed circuit boards to the fluid distribution manifold. Quick disconnect couplings provide an easy way to connect or disconnect the cold plates to the fluid distribution system when inserting or removing printed circuit boards or other assemblies.
While quick disconnect couplings are generally reliable, they inherently release a small amount of fluid each time a printed circuit board is removed, and inject a small amount of air each time a printed circuit board is inserted. Over time, the loss of cooling fluid and the introduction of air into a sealed cooling system can become a significant problem. For a two phase cooling system to work properly, some portion of the cooling fluid must always be in a liquid state. Otherwise, no liquid is available to absorb heat through a phase change and the electronic system will likely overheat.
The presence of air in a two phase cooling system, even when sufficient cooling fluid is available in its liquid state, can also lead to significant problems. Air is considered a “non-condensable” and impedes heat transfer particularly in the condenser component of a two-phase cooling system. Further, the addition of non-condensables increases the pressure within the fixed-size closed-loop cooling system. The two-phase cooling system maintains the electronics at a temperature proportional to the boiling point of the cooling fluid. Boiling point is directly related to the pressure in the system. As the amount of non-condensables in the sealed system increases, the boiling point of the cooling fluid therefore increases. Thus as air is introduced into the closed loop cooling system, the operating temperature of the electronic devices increases. If sufficient air is introduced into the cooling system, the electronic system may reach a temperature that is outside its acceptable operating range.
To avoid problems caused by the introduction of air, conventional two-phase cooling systems are hermetically sealed and do not allow fluid lines to be repeatedly connected and disconnected. The consequence on usage for cooling electronics is that to remove a printed circuit card the cold plates must now be detached. This results in a very lengthy and cumbersome process because of the number of required fasteners involved with cold plate attachments, requirements for the proper application of the thermal interface material that resides between the cold plate and the electronics components, and the precision alignment requirements.
A means to allow for the use of cold plate would be to utilize a pressure-relief valve that vents any excessive pressure due to induced air to the outside atmosphere. The drawback just as before for single-phase cooling systems is that a mixture of air and vapor will leave the system rather than solely the undesired air. The negative consequence is that the fluid amount will decrease over time.